The present invention relates generally to a nuclear reactor valve and more particularly to a breeder reactor blanket fuel assembly coolant system valve which increases coolant flow to the blanket fuel assembly to minimize long-term temperature increases caused by fission of fissile fuel created from fertile fuel through operation of the breeder reactor.
Valves may be used for many applications in nuclear reactors. Currently an important use of valves is in the nuclear reactor coolant system. However, no self-actuating valves are presently used to control coolant flow to each of the many fuel assemblies which form the core of the reactor. The present state-of-the-art uses a fixed-size orifice in each fuel assembly to provide the entrance for coolant flow to the fuel rods contained therein, and use of a check valve to prevent reverse flow has been considered.
In certain circumstances, varying the size of each fuel assembly coolant entrance orifice may be desirable. For example, in breeder reactors the blanket fuel assemblies experience a long-term increase in temperature due to fuel rod power increase caused by an increase in fissile fuel content. This is brought about by the breeder reactor's operation in converting the blanket fuel assemblies' fertile fuel into fissile fuel. The long-term temperature increase may be different for each fuel assembly. Blanket fuel assemblies are designed to operate within a certain temperature range. Higher temperatures will degrade the fuel assembly by shortening material life. Lower temperatures will degrade the reactor's performance by lowering its power for a given coolant flow. Thus, the inherent problem of breeder reactor blanket assemblies is that, with a fixed-size coolant entrance orifice, beginning-of-life temperatures are too low with acceptable end-of-life temperatures, or end-of-life temperatures are too high with acceptable beginning-of-life temperatures. A size-varying orifice valve could increase coolant flow to the blanket fuel assembly to keep long-term temperature increases to a minimum.
Another example where a size-varying fuel assembly coolant entrance orifice may be desirable is in a fissile fuel assembly designed for long life, where the fuel rod power, and hence temperature, will decrease as more of the fissile fuel is depleted over the long-term operation of the nuclear reactor. Here a size-varying orifice valve could decrease coolant flow to minimize temperature decreases.
Some ways of changing the size of the fuel assembly's coolant entrance orifice include shutting down the reactor to change the orifice unit with one of different size, or equipping the fuel assembly coolant entrance with an externally controlled valve. A mechanically or electrically actuated valve for each fuel assembly making up the nuclear reactor core would pose serious design and operation problems because of the hostile environment. Also, a self-contained temperature-actuated valve would follow the short-term temperature fluctuations of plant startup, power transients and plant shutdown, and pose time-lag and fail-safe problems.